Deciphering the nature of ion-graphene interaction

Abstract

Ion-specific interactions with materials are important for the design of ion-based energy and information devices, which are studied here with a focus on alkali-earth-metal and transition-metal ions in the context of carbon nanofluidics. Density functional theory calculations show that for the alkali-earth-metal ions, the nature of interaction is ionic, with charge transfer to and from graphene determined by the change in work function. In contrast, covalent bonding is identified for transition-metal ions adsorbed on graphene, showing hybridization near the Dirac point. The ion-graphene interaction is significantly stronger than van der Waals interactions and shows much higher contrast among the ions. We find that, surprisingly, these features are robust even with the presence of water solvation, where alkali-earth-metal ion-graphene interaction remains ionic with modified charge transfer, while covalent interaction with transition-metal ions turns into ionic. These understandings imply that the ion-wall interaction can be notably tuned by the external electric field, as verified by our calculations, and opens an avenue for the development of nanofluidics where the channel width is comparable with the range of ion-wall interaction.